干旱区生态环境知识资源中心

The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in and or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (<60 mu m). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai-Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the takes and ponds shows a clear response to lake water delta(18)O values, and these vary with water salinity. The relationship between lake water delta(18)O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small ’vital offsets’ of ostracods. Finally, the effect of water depth on temperature leads to increasing delta(18)O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface.

For arid, high-altitude Lake Qinghai, our results suggest that variations in the delta(18)O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive delta(18)O values of ostracod and bulk carbonate. (C) 2009 Elsevier B.V. All rights reserved.